Device for treatment of exhaust of an internal combustion engine

ABSTRACT

A method and a device for treatment of the exhaust of an internal combustion engine in which a fluid is used as an auxiliary agent for the treatment; a partial chemical conversion of the auxiliary agent is at least occasionally stimulated in order to produce a substance that reduces the freezing point of the fluid when the temperature of the fluid falls below a critical value.

PRIOR ART

The invention is based on a method and a device for treatment of theexhaust of an internal combustion engine, as generically defined by thepreamble to the independent claims. A method and a device of this kindare already known from DE 199 35 920, in which in order to prevent aurea/water solution from freezing at −11° C., heating tubes are providedin the reducing agent reservoir so that the reducing agent reservoir canbe heated when reducing agent temperatures fall below 20° C.

ADVANTAGES OF THE INVENTION

The method and device according to the present invention, with thecharacterizing features of the independent claims, have the advantageover the prior art of achieving a reduction in the freezing point of thefluid through concerted use of a conversion reaction of the auxiliaryagent, in particular a decomposition reaction, without having to acceptan appreciable temperature increase in the fluid system. At low outsidetemperatures, constant reheating is no longer necessary because after aconcerted chemical conversion, ice no longer forms, even at lowtemperatures, and the heating does not have to be activated as long asthe fluid contains a sufficient concentration of the substance producedby the conversion reaction.

Advantageous modifications and improvements of the methods and devicesdisclosed in the independent claims are possible by means of the stepstaken in the dependent claims.

It is particularly advantageous to carry out the stimulation in apartial region of the fluid volume contained in the tank and/or in linesso as to effectively enrich the fluid with the substance, withoutappreciably increasing the average temperature of the fluid.

It is easy to use the concerted decomposition of urea into ammonia toprevent the danger of freezing and thus the attendant risk of damage tolines and/or other system components, despite low outside temperaturesand despite only an insignificant increase in the temperature of thefluid on average chronologically and spatially.

Other advantages ensue from other characteristics disclosed in thedependent claims and mentioned in the specification.

DRAWING

Exemplary embodiments of the present invention are shown in the drawingand will be explained in greater detail in the subsequent description.

FIG. 1 shows a system for selective catalytic reduction of nitrogenoxides in the exhaust of an internal combustion engine.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In the sole FIG. (1), the exhaust line 9 represents the exhaust line ofan internal combustion engine, in particular a diesel engine of a motorvehicle. The exhaust 16 flows from the internal combustion enginethrough the exhaust line 9, passes a urea/water solution line (UWS line)8 connected to the exhaust line 9, and finally arrives in a catalyticconverter, not shown in detail, for selective catalytic reduction ofnitrogen oxides contained in the exhaust. Downstream of the SCRcatalytic converter (“SCR”=“selective catalytic reduction”), the exhaustflows through other devices, not shown in detail, for example othercatalyzing units and/or a muffler, into the open-air. At the endopposite from the region feeding into the exhaust line, the UWS line 8is connected to a urea/water solution tank 10. Between the tank 10 andthe exhaust line 9, the line 8 contains a pump 6 and, between the pump 6and the exhaust line 9, a metering valve 7 that can be cyclicallytriggered. The tank 10 contains a urea/water solution (UWS) with a ureacontent of for example 32.5% by weight. In a partial volume 13 of thetank 10 that is disposed in the lower region of the tank in the currentexemplary embodiment, an electrical heater 3 is provided; the supply ofelectrical power to the heater of the UWS in the partial volume isschematically indicated by the letters P_(EL). On the upper sideoriented toward the surface of the urea/water solution in the tank, aseparating element 2 that is fastened to the side region of the tankforms an upper boundary of the heater 3, which is embodied as anelectric heating coil. This boundary serves to define the region inwhich a significant heating or temperature increase of the fluid in thetank can occur. Lateral to the separating element and adjacent to it, atemperature sensor 4 and an ammonia sensor 5 are provided in order todetermine the temperature and the ammonia concentration in the heatablepartial volume. Above the fluid level of the urea/water solution, thetank 10, which can be closed by means of a closure device not shown indetail, is equipped with a pressure relief valve 11 that allows excessgas pressure to escape via a washing bottle 12 to which it connects. Inaddition, an electronic control unit 14 is provided that calculates,among other things, the intrinsically known functions of metering theurea/water solution into the exhaust train as a function of engineand/or exhaust parameters that are supplied to the control unit in amanner not shown in detail after being measured in the engine or exhausttrain. In addition, this control unit 14 is connected to the temperaturesensor 4 in order to evaluate a temperature signal 4 a and is connectedto the ammonia sensor 5 in order to evaluate an ammonia concentrationsignal 5 a. A control signal line 3 a can, for example, trigger a powertransistor circuit, not shown in detail, in order to regulate theelectrical heating capacity of the electric heater 3.

The pump and the metering valve 7 are triggered via triggering lines,not shown in detail, that are connected to the control unit 14 to supplya urea/water solution in an intrinsically known fashion to the exhaust16 in a metered form in order to achieve, in the NOx-reduction catalyticconverter not shown in detail below, a reduction of the nitrogen oxidescontained in the exhaust in accordance with the method of selectivecatalytic reduction. In this connection, within the exhaust train,ammonia is derived from the urea/water solution supplied to the exhausttrain and this ammonia selectively reacts with the nitrogen oxides inthe SCR catalytic converter to produce nitrogen and water. In additionto the complete conversion of the urea/water solution into ammonia inthe region of the exhaust train, in a partial region of the urea/watersolution system comprised of the tank 1, line 8, pump 6, and meteringvalve 7, according to the present invention, the urea/water solution inthe partial volume 13 of the tank 10 is heated to a decompositiontemperature in a range above 60° C. in order to stimulate an occasional,limited, partial decomposition reaction of urea to produce ammonia. Thecontrol unit 14 controls the heating capacity and the activation periodof the heater as a function of the temperature and ammonia concentrationvalues measured in the partial volume 13. This supply of heat occurswhen a critical temperature value is reached or exceeded, which lies ina range from 0° C. to −11° C., preferably in a range from −5° C. to −10°C. In this connection, care is taken to assure a sufficiently highammonia concentration in the entire fluid volume of the urea/watersolution system in order to achieve a sufficiently significant reductionin the freezing point so that a subsequent reheating can be avoided evenif the temperature falls below the critical value again. However, if theammonia concentration has decreased to the point that a sufficientreduction in the freezing point is no longer assured, then the heatingmust be switched on again when the temperature falls below the criticaltemperature value. The ammonia concentration normally fluctuates in arange of between 7 and 20 percent, thus yielding a freezing pointreduction in a range from 10K and 50K. It is particularly advantageousto establish a value of approx. 7 to 15 percent by volume of ammonia inthe urea/water solution in order to reduce the freezing point of theurea/water solution from −11° C. to a range from −20° C. to −30° C. Thetemperature of the urea/water solution is raised by 5K to 50K on averagechronologically and spatially, and the pressure in the tank increasesonly slightly. The pressure relief valve 11 blows off any excesspressure generated by the escape of ammonia from the urea/watersolution. Before the excess pressure is released into the environment,the gas passes through the washing bottle 12, by means of which theammonia contained in the gas can be removed from the escaping gas inorder to minimize the risk of environmental damage due to escapingammonia. Up to 700 l of ammonia can be dissolved in 1 l of water. In thecurrent exemplary embodiment, the tank volume is 60 l, so that atatmospheric pressure, 10 times the tank volume in pure ammonia can bedissolved in 1 l of water contained in the washing bottle 12. Either thestate of the water in the washing bottle is monitored by means ofdevices not shown in detail or the water is replaced at regularintervals (for example after a fixed number of tank refills).

In an alternative embodiment form, the escape of excess pressure in thetank 10 is not conveyed into the environment, but rather into theexhaust line in order for ammonia that may have passed through thewashing bottle 12 to still be neutralized in the SCR catalytic converterand to assure an additional degree of certainty in preventingenvironmental damage. In an improved embodiment form, the method for theconcerted decomposition of urea into ammonia in the region of the UWSsystem for freezing point reduction can include an additionalmeasurement of the outside temperature so that a freezing pointreduction is activated only when necessary, i.e. at low outsidetemperatures, particularly in the subzero range. The heating of apartial region of the UWS volume and the stimulation of a decompositionof urea into ammonia in a partial region of the UWS system can also becarried out in a part of the system other than the tank. When using ametering valve 7 in the form of an injection valve cooled with theurea/water solution, it is also possible to powerfully throttle thecoolant flow of the urea/water solution so that the urea/water solutionused as the coolant is heated in the desired fashion to stimulate theurea decomposition. The ammonia sensor here can, for example, bedisposed in the tank while the temperature sensor is disposed in thecoolant flow. This alternative, not shown in detail, comprised of alsousing the urea/water solution as a coolant for cooling the meteringvalve disposed in close proximity to the exhaust train while providing athrottling of the coolant flow, can be further improved by embodyingthis throttling as controllable, in particular so that it can becontrolled as a function of measured urea/water solution temperatures ormeasured ammonia concentration values. Furthermore, in anotherembodiment form, the pressure relief valve 11 can be designed so as topermit a definite pressure increase in the system, which shifts thechemical equilibrium between urea and ammonia in the system further inthe direction of the ammonia gas. Alternatively, an auxiliary heatingsystem can also be used to locally heat the urea/water solution in apartial region of the tank 10 or in a partial region of the exhaust line8 in order to lighten the load on the vehicle battery, particularly whenthe stimulation of the decomposition reaction is to also occur duringlong stops, i.e. when the engine is switched off. In other alternativeembodiment forms, the method and the system can be embodied and/orimproved so that a temperature gradient that is produced causes a masstransfer to occur in the system, in particular between the heatedpartial region and the rest of the tank region in order to be able totreat the entire urea/water solution in relatively short periods of timethrough the local supply of heat and to assure a rapid distribution ofthe generated ammonia throughout the entire system. The better thesystem is designed to encourage such a convection, the less energy isrequired because the generated ammonia is immediately distributedthroughout the system and remains dissolved without immediately escapingfrom the aqueous solution and therefore being unavailable for reducingthe freezing point.

1-13. (canceled)
 14. A method for treatment of the exhaust of aninternal combustion engine in which a fluid is used as an auxiliaryagent for the treatment, the method comprising the steps of at leastoccasionally stimulating a partial chemical conversion of the auxiliaryagent in order to produce a substance that reduces the freezing point ofthe fluid when the temperature of the fluid falls below a criticalvalue.
 15. The method according to claim 14, wherein the conversion ofthe auxiliary agent is stimulated before the auxiliary agent isintroduced into the exhaust.
 16. The method according to claim 14,wherein the fluid is drawn from a tank and supplied to the exhaust vialines, and wherein the stimulation occurs in a partial region of thetank or in a fluid volume contained in the lines so that a sufficientquantity of the substance can be distributed in the fluid volume inorder to achieve a uniform freezing point reduction.
 17. The methodaccording to claim 15, wherein the fluid is drawn from a tank andsupplied to the exhaust via lines, and wherein the stimulation occurs ina partial region of the tank or in a fluid volume contained in the linesso that a sufficient quantity of the substance can be distributed in thefluid volume in order to achieve a uniform freezing point reduction. 18.The method according to claim 14, further comprising the step ofsupplying heat to produce the stimulation.
 19. The method according toclaim 16, further comprising the step of supplying heat to produce thestimulation.
 20. The method according to claim 17, further comprisingthe step of supplying heat to produce the stimulation.
 21. The methodaccording to claim 19, wherein heat is supplied for a time to heat thepartial region of the fluid to a temperature above 60° Celsius.
 22. Themethod according to claim 18, wherein due to a spatial distribution, thesupply of heat causes only a slight temperature increase in the fluidvolume over time.
 23. The method according to claim 21, wherein due to aspatial distribution, the supply of heat causes only a slighttemperature increase in the fluid volume over time.
 24. The methodaccording to claim 22, wherein the slight temperature increase lies inthe range between 5 Kelvin and 50 Kelvin.
 25. The method according toclaim 23, wherein the slight temperature increase lies in the rangebetween 5 Kelvin and 50 Kelvin.
 26. The method according to claim 14,wherein the freezing point is reduced by 10 to 30 Kelvin.
 27. The methodaccording to claim 14, further comprising the step of measuring theconcentration of the substance in the fluid and/or the temperature ofthe fluid, and establishing the intensity and/or duration of thestimulation as a function of the concentration of the substance and/orthe temperature.
 28. The method according to claim 16, furthercomprising the step of measuring the concentration of the substance inthe fluid and/or the temperature of the fluid, and establishing theintensity and/or duration of the stimulation as a function of theconcentration of the substance and/or the temperature.
 29. The methodaccording to claim 18, further comprising the step of measuring theconcentration of the substance in the fluid and/or the temperature ofthe fluid, and establishing the intensity and/or duration of thestimulation as a function of the concentration of the substance and/orthe temperature.
 30. The method according to claim 27, wherein theconcentration and/or the temperature is measured in the partial region.31. The method according to claim 14, wherein the substance is a gasthat is soluble in the fluid.
 32. The method according to claim 14,wherein a urea/water solution is used as the fluid and ammonia is thesubstance.
 33. A device for treatment of the exhaust of an internalcombustion engine in which a fluid (1) is used as an auxiliary agent forthe treatment, the device comprising means (2, 3, 4, 5, 3 a, 4 a, 5 a,14) for at least occasionally stimulating a partial chemical conversionof the auxiliary agent into a substance that reduces the freezing pointof the fluid, the means being disposed and/or embodied so as to permitthe stimulation to occur when the temperature of the fluid falls below acritical value.